1. Field of the Invention
This invention relates to a beam scanning device, and more particularly to a device for recording information on a recording medium of small recording area such as a microfilm by scanning the medium with a light beam.
This invention is particularly applicable to a system in which computer output information is to be recorded on a microfilm with high speed and high resolution.
2. Description of the Prior Art
It has been known in the art to use a collimated light beam such as a collimated laser beam in a high speed image recording device in a so-called COM system wherein computer output information is recorded on a microfilm. A laser beam can be advantageously used in recording information in a small area because of the highly collimated and converged small light spot obtained thereby. Further, by producing a small light spot of extremely high energy density by converging the laser beam, it is possible to use recording media of low sensitivity. Because of these advantages which make possible the recording of information on a recording media of low sensitivity with high recording density, laser beam scanning has recently been deemed the most convenient method for recording computer output information.
In the laser beam scanning system, a beam deflector is used to make the beam scan the recording medium. There have been know various kinds of beam deflectors such as beam deflectors utilizing acousto-optical or electro-optical effect and beam deflectors using a rotating multi-face mirror or a vibrating mirror such as a galvanometer. Among these beam deflectors, the beam deflector utilizing acousto-optical or electro-optical effect is disadvantageous because of its large loss of light and low resolution; it, however, is suitable for random access. The beam deflector using a vibrating mirror is disadvantageous in that the image is greatly distorted by the variation in speed of the scanning light spot as it scans different points on the rasters; on the other hand, the driving power source circuit thereof is simple and easily handled when the deflector is driven by a sine wave A.C. voltage source. When the deflector is driven by a rectangular wave voltage, it is difficult to obtain high speed in scanning.
In view of the above observations, beam deflectors using a rotating multi-face mirror are considered to be the best for use in a system for scanning microfilms with a laser beam. There are, however, the following great difficulties in manufacturing this type of beam deflector.
First, faces of the rotating multi-face mirror must be precisely processed to form equal angles therebetween. Errors in the angles between adjacent faces cause displacement of the rasters in the scanning direction.
Second, faces of the multi-face mirror must be precisely processed to be in parallel to each other. Errors in the parallelism of the faces cause displacement of the rasters in the direction perpendicular to the scanning direction and cause irregularity in the pitch of the rasters.
Third, faces of the mirror must be precisely in parallel to the axis of rotation of the rotatable shaft of the motor to which the rotating mirror is mounted. Errors in the parallelism between the faces of the mirror and the axis of rotation cause displacement of the rasters in the direction perpendicular to the scanning direction.
The above described three difficulties relate to the precision required in the process of manufacturing the rotating multi-face mirror and in installation of the mirror to the driving motor. In order to keep processing and installation errors small, a highly precise and accordingly costly manufacturing process is required. Therefore, it is desirable to compensate for these errors by a method other than the method for improving the preciseness in manufacture.
The above mentioned errors deriving from the first difficulty concerning the angles formed between adjacent faces of the multi-face mirror can be comparatively simply corrected by an electric process which uses a part of the deflected beam coming from the multi-face mirror as a synchronizing signal for driving a video signal. The first difficulty is, therefore, not serious. On the other hand, errors deriving from the second and third difficulties cannot be simply corrected. For instance, in order to correct these errors by an electric process, the errors in the parallelism of faces of the mirror must be memorized in a recording medium and the incident angle of the beam impinging on the multi-face mirror changed to cancel the displacement of the rasters by use of a beam deflector driven by a signal from the recording medium. This electric process for correction requires a memory or recording medium for recording errors and a beam deflector for correcting the incident angle of the scanning beam, which complicate the construction of the beam scanning device.